1. Field of the Invention
The present invention relates to an optical disk apparatus and a method for processing a playback signal in the optical disk apparatus.
2. Description of the Related Art
One of distinguished features of an optical disk apparatus and a disk compliant with standards of DVD, Blu-ray Disc (BD) and the like is that a recording medium is commutative and recording and playback can be performed even when disks are exchanged between different apparatuses. Compatibility is an extremely important matter for an optical disk, and requirements for the compatibility are set as the standards. However, in reality, the compatibility may be lowered for various reasons. For example, recording may be performed in a state where laser power in recording widely deviates from its proper value for some reasons. Moreover, a surface of a disk recorded is scratched or stained and, as a result, the compatibility may be lowered.
In order to maintain the compatibility, it is needed not to make a disk which cannot or can hardly be played back by other apparatuses, when recording in the disk. Moreover, in playback, it is required that devices are made so as to hinder a read error from occurring as much as possible, even if a disk is in a bad condition including the case where the disk is defective or stained. Therefore, to improve playback performance of the optical disk apparatus is an essential requirement for enhancing the compatibility. Thus, playback ability has been enhanced by introducing various technologies, such as an adaptive target Viterbi decoder (synonymous with an adaptive Viterbi decoder), described in Japanese Patent Laid-Open No. Hei 11 (1999)-296987, for example, for dealing with the case where a playback signal has large asymmetry.
Moreover, except for the time of stream playback and the like, generally, when a read error occurs, a read retry is performed in order to acquire data in the cluster. In the read retry, trials have heretofore been done according to a pre-prepared retry parameter list without specifying a factor for a read error.
As a result of improving the playback performance of the optical disk apparatus, when confirmation playback (verify) after recording is performed under the same conditions as those of normal playback, detected symbol errors are few, even thought recording quality is significantly deteriorated by problems in recording. Accordingly, there will be more and more cases where the deterioration thereof cannot be identified. Consequently, in the case where a recording region of the disk is played back by different apparatuses, the read error is more likely to occur. That is, the compatibility is lowered.
Moreover, there is a wide variety of events which trigger the read error. In many cases, the read error is caused by an exceptional event. In the case where a degree of the error is minor, if playback is tried again under normal playback conditions, the playback is successfully performed with a considerable probability (simple retry). However, in the case where the degree thereof is major, a sufficiently high probability of success may not be expected unless drive operation parameters are changed at the time of read retry. In addition, a host which issues a read command generally determines that it is unable to read if the host cannot acquire data within a certain period of time after issuance of the command (timeout). Thus, the number of times of the read retries which can be executed is limited.
The major factors for the read error can be classified broadly into three groups, including a defect, a deviation and a SNR deficit. Moreover, there are differences in setting policies of playback parameters in the read retries corresponding to the respective factors. In the read retry, trials have heretofore been done according to a pre-prepared retry parameter list without specifying the factor for the read error (such a parameter list will be hereinafter called a retry list). Thus, in some occasion the trials have been done by using parameters which are absolutely invalid, or even have adverse effects. Moreover, since there is a limitation on the number of trials which can be executed, there is a problem that variations on effective trial parameters are limited. To be more specific, if the deviation is a cause of the read error, it is effective to increase or decrease a tracking or focusing gain, and to lower a play back speed. Meanwhile, if the defect is a cause of the read error, the procedures described above are more likely to have adverse effects.